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In reference to the Kepler 22b news:

The Kepler team had to wait for three passes of the planet before upping its status from "candidate" to "confirmed".

This is possible because the planet has a orbital period ('year') of 289 days, and so it has transitioned more than once (as above) since the Kepler mission started.

However I've seem some confirmed exoplanets that have orbital periods in 10s and 100s of years, and therefore could only have been detected once by Kepler.

As far as I'm aware all methods of discovering exoplanets require the planet to pass in from of their parent star (i.e. we can't use a telescope to 'zoom in' and see them directly)?

So how are exoplanets confirmed, when we haven't been searching long enough to cross in front of their parent star? How long does the transition even last?

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Don't know, but I imagine that for slow transits you have a better impinging and expinging(?) light curve, and a better chancing of doing spectroscopy on any atmosphere the planet may have. Perhaps enough certainty can be found from some other analysis. –  dmckee Dec 5 '11 at 22:46
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I could answer your question until you stated that you've seen some confirmed exoplanets with periods 10-100s of years. Could you provide a source for that? –  Stuart Robbins Dec 6 '11 at 0:07
    
Sorry I had forgotten about the wobble of stars, how stars with 100s of yrs are found, but still there are some transiting discoveries that have orbital periods longer then the mission eg 55 cnc d (simbad.u-strasbg.fr/simbad/…) –  Jonathan. Dec 6 '11 at 0:55
    
@dmckee impinging/expinging = ingress/egress ! –  Nic Dec 7 '11 at 12:52
    
@dmckee Exoplanet spectroscopy is still more of a dark art than a science (not that I'm against it - it just needs time to mature). And light curve quality is really determined by whether you're using Hubble vs. ground-based telescopes more than anything else. Confirmation of long-period transits requires other techniques, and it will be many years before we've gone through the Kepler candidates. –  Chris White Oct 2 '12 at 8:13
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Though the definition of "confirmed" is not a firm one, and is really an operational matter of peer review and consensus among researchers, the most widely accepted method of confirmation is high-resolution radial velocity analysis: the observation of shifts in the spectrum of the parent star as it is alternately "tugged" towards and away from an observing telescope by a candidate planet.

The observations used for such analysis require large and sophisticated optical telescopes with sophisticated spectrographic equipment, so that only a relatively small number of the hundreds of exoplanet candidates detected by transit surveys such as the Kepler Mission can be confirmed in this way. Additional confirmation methods include observation of additional transits, and (in rare cases) direct imaging. As the questions correctly notes, observation of multiple transits is constrained by their frequency, so that planets with very long orbital periods require a (potentially quite long) time to confirm in this way. In practice, these multiple lines of evidence (along with careful modeling, knowledge of stellar history and variability, etc.) are taken together to build a case for the "confirmation" of an exoplanet.

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